processing of E6 and B&W films were done by an outside company called CEWE. The stores themselves should be around for a little while, so perhaps go in and ask. If a "
slide film mailer" (I have no idea what this is btw
) is a prepaid service, you can safely bet that they wont honour it. Otherwise, the turn around time for most 'dev by mail' services that jessops offer/offered is about 2-3 weeks.